The number of ions produced as a function of the energy of the electrons or photons used to produce ionization.
Orange Book, 2nd ed., p. 206 (http://media.iupac.org/publications/analytical_compendium/)
PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1548 (https://doi.org/10.1351/pac199163101541)